Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов

Figure 11 Contributions of Fe²⁺ (light gray) and Fe³⁺ (dark gray) to the Mössbauer spectrum of a Fe‐containing borosilicate glass as determined from fits made with 2‐d Gaussian distributions [15].

      6.3 Ultra Violet/Visible Spectroscopy

      The absorption, reflection, or emission of light in the near UV to near IR (~250–3000 nm) is also a source of structural information. In UV/Vis spectroscopy, one thus measures the absorption of light by a material caused by a number of factors: electronic transitions of transition metal d electrons, intervalence charge transfer (IVCT) and anion–cation charge transfer, electronic transitions between the conduction and valence bands, vibrational overtones, electronic transitions between f‐orbitals, defects, and electron holes [26]. The technique is primarily used to investigate the causes of color in glasses through the oxidation and coordination environment of coloring transition metal elements such as Ni within glasses (Chapter 6.2). In general, specific absorption bands are observed that are characteristic of the transition metal, its oxidation state, and coordination. The samples are usually polished glass chips (mm) or slabs (mm–cm). In addition, sample thickness may need to be adjusted for specific experimental conditions.

Figure 12 The different oxygen species identified from the XPS O 1s spectrum of a lead silicate glass. BO peak is constrained to a linewidth of 1.22 eV and fitted with two peaks in the NBO envelope, one due to NBOs and the other to the presence of free oxygen. Glass made and spectrum obtained by Ryan Sawyer, University Western Ontario.

7 Perspectives

      In this chapter I have by no means covered all the possible methods that can be used to investigate the structure of glasses. In addition to variations in many of the specific techniques covered in this chapter, many more techniques including imaging methods such as atomic force microscopy (AFM) and high‐resolution transmission electron microscopy (HRTEM) can be used (Chapter 2.3). Whereas the ability to “solve” the structure as done in crystal‐structure analysis is not possible, our ability to probe the structure of glasses has greatly improved since the first studies in the early decades of the twentieth century. Progress in X‐ray and neutron sources, lasers, detector sensitivity and resolution, and computers have all contributed to improve greatly the resolution and length scales observable in glasses and other amorphous materials.

      Unlike for crystalline materials, however, any structural study of a glass must involve a multi‐technique approach since no single method can supply information on all aspects of the structure. In addition, it is advantageous to use numerical techniques such as simulations of spectra, first‐principles molecular dynamics (MD) simulations, and reverse Monte Carlo (RMC) calculations to aid interpretation of the experimental data. While our understanding of the SRO continues to improve, that of the IRO and LRO remains murky, as is the link between structure and, physical properties and behavior, as well as, structural changes that occur with increasing pressure and temperature. With continued advances in experimental instrumentation and computational methodologies, the outlook for resolving many of these structural issues is good.

Acknowledgements

      Thanks to P. Salmon for providing Figures 2a–c and constructive comments; D. Neuville for providing Figures 8 and 9; J.F. Stebbins, B.O. Mysen, L. Cormier, and G. Lelong for comments, suggestions, and keeping me on track with regard to the different techniques; and E.I. Kamitsos for providing me with a copy of his IR chapter prior to publication. Diffraction images provided courtesy of T. Höche of the Center for Applied Microstructure Diagnostics (CAM), Fraunhofer Institute for Mechanics of Materials IWM, Halle Germany.

References